The materials collectively referred to as rubber in diversified fields generally mean crosslinked rubber, which is obtained by mixing crude rubber with a filler, such as carbon black, or adducts, such as a crosslinking agent, and then crosslinking the crude rubber in the post-crosslinking step called as curing for the purpose of reinforcing the crude rubber. The post-crosslinking step is an indispensable step to confer mechanical strength, rubber elasticity, durability, etc. to rubber.
However, a process of producing the crosslinked rubber as a final product through the above step comprises numerous steps, and each step demands a great amount of energy and a considerable time, thereby raising the producing costs of the crosslinked rubber significantly.
To save the energy and time spent in the kneading step of kneading the crude rubber with the filler or adducts, a technique of lowering the viscosity of a blended product by adding adducts, such as lubricant, oil, low molecular weight compounds, to the crude rubber is adopted. However, when this technique is used, the adducts often give adverse effects to the physical properties of the crosslinked rubber. In case of liquid rubber, such as liquid polyisoprene and liquid polyurethane, the energy and time spent in the kneading step can be reduced to some extent, but there remains a problem that the cost of raw materials is high. Moreover, the post-crosslinking step is indispensable in this technique.
To solve the above problem, some processes which can simplify the crosslinking step have been proposed as an alternative of the above process of producing the crosslinked rubber from the crude rubber. For example, Japanese Laid-open Patent Application Nos. 74110/1981 (Tokukaisho No. 56-74110) and 179209/1982 (Tokukaisho No. 57-179209) disclose a process of producing ion crosslinked acrylic resin (acrylic rubber) by radical-polymerizing an acrylic-based monomer mixture containing a metal salt vinyl monomer. Since this producing process does not need the post-crosslinking step, crosslinked rubber (acrylic-based ion crosslinked elastomer) having excellent rubber elasticity and ductility can be produced in a simple procedure. In other words, since the acrylic-based monomer mixture containing the metal salt vinyl monomer, which is a crosslinking monomer, is radical-polymerized in the above producing process, the crosslinked rubber can be obtained in a so-called "1 pot" reaction.
However, according to the above conventional producing process, an amount of the metal salt vinyl monomer contained in the acrylic-based monomer mixture, that is, a concentration of the metal salt vinyl monomer, can be increased only to a certain limit. This means that, according to the above conventional producing process, if the crosslinking degree of the crosslinked rubber is increased, there is a limit for further improvements of the mechanical strength and other physical properties. Further, when the mechanical strength, such as tensile break strength and tensile break elongation, of the ion crosslinked rubber are improved (upgraded), the creep in compression set, tensile elongation set, etc. is undesirably increased in turn. Consequently, the crosslinked rubber obtained by the above conventional producing process has a problem in the physical properties that its creep becomes as large as the creep of non-crosslinked rubber.
On the other hand, since the acrylic rubber has excellent physical properties, such as heat resistance, oil resistance, and weather resistance, it can be used suitably and extensively as sealing materials, packing materials, hose materials, for automobiles, civil construction, marine vessels, etc. As disclosed in Japanese Laid-open Patent Application Nos. 312339/1988 (Tokukaisho No. 63-312339) and 218704/1990 (Tokukaihei No. 2-218704), the acrylic rubber is produced by subjecting crude rubber, obtained by suspension-polymerizing or emulsion-polymerizing an acrylic-based monomer, to the post-crosslinking (curing) through various techniques to improve the mechanical strength and rubber elasticity.
Nevertheless, the acrylic rubber is inferior to general rubber in the mechanical strength, such as tensile strength and tear strength. Also, when the acrylic rubber is produced, the post-crosslinking step using a crosslinking agent and the kneading step of kneading the crude rubber with a filler or adducts, such as a reinforcing material, are indispensable. Thus, the acrylic rubber is disadvantageous in workability and productivity.
Recently, to eliminate the drawbacks of the acrylic rubber and the producing process thereof, various kinds of acrylic-based elastomer of an ion crosslinking type containing metal salts have been proposed. A producing process of the acrylic-based ion crosslinked elastomer is disclosed in, for example, aforementioned Japanese Laid-open Patent Application Nos. 74110/1981 (Tokukaisho No. 56-74110) and 179209/1982 (Tokukaisho No. 57-179209), in which a monomer mixture, made of alkyl (meth)acrylate and metal salts of polymerizable unsaturated carboxylic acid, is radical-polymerized.
However, in the above conventional producing process of the acrylic-based ion crosslinked elastomer, it is difficult to dissolve a relatively large amount of metal salts of polymerizable unsaturated carboxylic acid into alkyl (meth)acrylate, in other words, it is difficult to raise a concentration of the metal salts in the monomer mixture to a relatively high level. Thus, although the acrylic-based elastomer can be produced by a simple procedure, the crosslinking degree of the same is unsatisfactory. Consequently, the resulting acrylic-based ion crosslinked elastomer has the same problem as the acrylic rubber that it is inferior in mechanical strength, such as tensile strength and tear strength. In addition, the acrylic-based elastomer of the ion crosslinking type with the improved mechanical strength generally has larger creep in the compression set, tensile elongation set, etc., and therefore, its rubber elasticity is unsatisfactory. Thus, the conventional acrylic-based ion crosslinked elastomer has a difficulty in improving both the mechanical strength and physical properties, such as creep, in a well balanced manner, and for this reason, the acrylic-based ion crosslinked elastomer has not been put into practical use yet.
A relatively large amount of metal salts of polymerizable unsaturated carboxylic acid can be dissolved into alkyl (meth)acrylate with the use of a solubilizing agent. In this case, however, the performance (physical properties) of the resulting acrylic-based ion crosslinked elastomer is deteriorated by the solubilizing agent. When an amount of metal salts of the polymerizable unsaturated carboxylic acid contained in the monomer mixture is merely increased, although the mechanical strength and the like can be improved, the creep in the compression set, tensile elongation set, etc. can not be reduced.